2. Grill or broil mushrooms until charred and tender, about 4 to 6 minutes per side. Remove from grill. While mushrooms are cooking, place rolls cut side down on grill or in a toaster oven and lightly toast, about 1 minute.

3. Use a quarter of the Boursin for each sandwich, spreading it on the two halves of each ciabatta roll. Thinly slice mushrooms and mound on bottom half of roll, followed by a quarter of the arugula or watercress. Cover with top of roll, cut sandwich in half on the diagonal, and serve.